A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for a traditional internal-combustion engine used in modern vehicles.
A common type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell includes three basic components: a cathode, an anode and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrode-assembly (MEA). The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen from air, for an electrochemical fuel cell reaction. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of generating a quantity of electricity sufficient to power a vehicle.
During periods of non-operation, a quantity of air diffuses into the fuel cell stack and accumulates on the anodes. Upon a start-up operation of the fuel cell stack, hydrogen is supplied to the anodes. The fuel cell stack may be purged with hydrogen, for example, as disclosed in assignee's copending application Ser. No. 11/762,845, incorporated herein by reference in its entirety. The hydrogen displaces the accumulated air and creates a “hydrogen-air front” that travels along the anodes. The hydrogen-air front is known to degrade the fuel cells and impact fuel cell stack performance. In particular, the presence of both hydrogen and air on the anodes results in a localized electrical short between a portion of the anodes that have hydrogen and a portion of the anodes that have air. The localized electrical short causes a rapid corrosion of the carbon support on which the catalyst is disposed. The rate of carbon corrosion has been found to be proportional to a time that the hydrogen-air front exists and a magnitude of the localized voltage at the hydrogen-air front. The carbon corrosion reduces the useful life of the MEAs in the fuel cell stack.
It is known in the art to short circuit the fuel cell stack during the start-up operation to minimize the voltage generated by the hydrogen-air front. A typical system and method for shorting the fuel cell stack is disclosed in assignee's copending application Ser. No. 11/858,974, now U.S. Pat. No. 7,807,308, incorporated herein by reference in its entirety. In the typical shorting system, an electrical load is used to minimize the localized voltage during the start-up operation. However, such systems generally require additional system componentry and may be volumetrically difficult to package in an engine compartment of a vehicle.
There is a continuing need for a shorting system that is volumetrically efficient, less massive, and that employs existing componentry of the fuel cell system for shorting the fuel cell stack. Desirably, the shorting system enables a method that minimizes stack degradation by shorting the fuel cell stack during the start-up operation.